1. Field of the Invention
The present invention relates to an object lens for an optical pickup, an optical pickup, and an optical information processing device.
2. The Description of the Related Art
For “an optical recording medium” such as a CD (compact disk) or a DVD (digital video disk), “an optical information processing device” which performs recording, reproduction, and deletion of information using an optical pickup has been spread widely. In this technical field, high-density recording onto the optical recording medium is demanded.
Since the light spot formed on the recording surface of the optical recording medium is formed of a beam waist of a laser beam which converges by means of the object lens of the optical pickup, and the diameter of the beam waist is proportional to the wavelength of laser beam, and, also, is inverse proportional to the numerical aperture (NA) of the object lens, increase in NA of the object lens in the optical pickup and shortening in the wavelength of laser beam are demanded accordingly.
As to the shortening of the wavelength of laser beam, a semiconductor laser having “the emission wavelength around 660 nm” for DVD is already put in practical use, and, these days, a laser light-source having the wavelength around 400 nm is also being put in practical use.
As the object lens having the large NA exceeding 0.7 has been proposed (for example, see Japanese laid-open patent application No. 10-123410). However, any of such object lenses having the large NA has a two-lens-combined configuration. Compared with a single-lens configuration, such a type of lens of two-lens-combined configuration requires complicated assembling process, high precision in assembling work, and, also, may not be driven at a high speed due to increase in the weight itself.
Moreover, as a working distance, i.e., the distance between the object lens and the optical recording medium surface is shortened in case of employing such a two-lens-combined configuration, there increases a risk of hitting of the object lens onto the recording medium surface, resulting in a series damage occurring thereon. As a result, it becomes difficult to achieve high reliability in this scheme.
Although such a problem does not occur in case of usage of a single lens configuration in the object lens, it may not be possible to achieve large NA. For example, around 6.5 is the maximum on those known.
Conventionally, an incident side substrate thickness, i.e., the distance between the surface of the recording medium on which the beam is incident and the recording surface thereof is prescribed as being 1.2 mm for CD while as being 0.6 mm for DVD. However, recently, a new trend occurs in which the incident side substrate thickness is to be standardized into as small as 0.1 mm.
There is a variation for every individual in the emission wavelength of the semiconductor laser generally used as the light source of the optical pickup, and there is also a phenomenon called mode hopping in which several nm change in the main wavelength occurs with temperature change etc. in the semiconductor laser.
In the optical pickup, when the wavelength emitted from the semiconductor laser changes from the original design value due to the variation for every individual, or due to the mode hopping, chromatic aberration arises in the optical pickup's optical system, the diameter of beam spot increases on the medium recording surface, and thus, there is a possibility of causing a problem on recording/reproducing operation in the device.
Especially the chromatic aberration occurring in the object lens by the wavelength change when using the short wavelength semiconductor laser not more than the emission wavelength around 440 nm may causes a nonpermissible problem. Namely, the refractive-index change with respect to a minute wavelength change becomes large in such a short wavelength range, and also, the chromatic aberration becomes large and the amount of defocus which is the focal movement amount becomes large. Furthermore, the beam spot on the recording medium is shortened so as to achieve high density recording, and, also, the focal depth of the object lens is proportional to the wavelength and inverse proportional to the second power of NA. Accordingly, the focal depth becomes smaller as the wavelength is shorter, and, thus, the tolerance on the defocus becomes restricted.
On the other hand, manufacture error on the order of ±10 nm is not avoidable with respect to the incident side substrate thickness of 0.1 mm. Such a substrate thickness error may cause spherical aberration in imaging function of the object lens designed according to the standardized substrate thickness. Thereby, the beam spot diameter may be increased, and, thus, the proper operation may not be expected in the optical pickup. As well-known, the spherical aberration is proportional to the forth power of NA of the object lens, the substrate thickness error may cause a larger problem as NA of the object lens increases.
As the optical recording medium, recently, in order to achieve a large recording capacity, a so-called “multilayer optical recording medium” in which a plurality of recording layers are placed on each other in a single recording disk has been put in practical use. In such a multilayer optical recording medium, since the recording surface separation (space thickness) of several 10 micrometers or more is needed and the distance from the object lens is different for every recording surface when recording, reproduction, or deletion of information is performed independently onto the respective ones of the plurality of recording surfaces. Accordingly, the spherical aberration may occur on a recording surface thereof different from the optimum position.
In the semiconductor laser used as the light source, utilization of the semiconductor laser with an oscillation wavelength around 400 nm has been attained. As the high NA lens, the high NA lens for the pickup which includes an aspheric surface lens of two-lens-combined configuration is disclosed in Japanese laid-open patent applications Nos. 2001-83410,11-202194, and 11-203711. However, as described above, such a two-lens-combined configuration object lens even having high NA may be problematic in comparison to a single lens configuration.
Japanese laid-open patent application No. 2001-324673 discloses an object lens beyond NA: 0.7 of single lens configuration solving the above-mentioned problem.
However, these conventional examples have a low implementability in terms of manufacture. In fact, in order for the object lens of single lens configuration to attain a raise in NA, and shortening the applicable wavelength, it is necessary to select such a type of glass material that press fabrication thereof can be made using a die having ultra-precision machining performed thereon, it is necessary to achieve satisfactory wavefront performance at a design median, and, also, it is necessary to make the manufacture tolerance to fall within a range of implementability. Specifically, first, as to the wavefront aberration in the design median, it should be controlled less than 0.01λ.
According to the inventor's calculation, the wavefront aberration of the object lens for an optical pickup disclosed by Japanese laid-open patent application No. 2001-324673, embodiment 3 should be 0.037λ, and thus, we could say that implementality is low, where this object lens is used for the wavelength of 400 nm, has NA: 0.85, and f (focal length): 1.765 mm, nd (refractive index in the lens material with respect to the d-line): 1.71667, and νd (number of Abbe in the lens material with respect to the d-line): 53. Moreover, actual utilization thereof is difficult when the implementability on the condition of the manufacture tolerance is low even when the wavefront performance in the design median is satisfactory. For example, the wavefront aberration needs to fall on the order of not more than 0.015λwith respect to the deviation in the thickness more than ±1 μm. FIG. 1 shows a relationship between the thickness tolerance and wavefront aberration on the object lens for optical pickup used in the wavelength: 650 nm, having NA: 0.75, f: 2.00 mm, nd: 1.69330, and νd: 53, as another example. As can be seen therefrom, the above-mentioned conditions cannot be actually satisfied.
Therefore, it is demanded to provide a lens with a reduced amount in wavefront performance degradation with respect to the manufacture tolerance, or to provide an optical pickup which can control the amount in wavefront performance degradation with respect to the manufacture tolerance.
Moreover, while the new standard of increasing NA and shortening the applied wavelength will be issued near future, there exist the conventional CD and DVD. It is preferable that these conventional optical recording media and the optical recording media according to the above-mentioned new standard can both be dealt with by a common optical information processing device. As an easier method therefor, a conventional optical pickup and an optical pickup according to the new standard are both mounted in one information processing device. However, it is difficult to attain a miniaturization and cost reduction of the device according to such a scheme.